Protective coatings are extensively used to protect metallic substrates, such as steel pipes and pipelines, from corrosion and mechanical damage. Widely used commercially-available coatings for such substrates include fusion bonded epoxy coatings. A typical process for producing a fusion bonded epoxy coating is described in U.S. Pat. No. 3,904,346 (Shaw et al), and involves the electrostatic spraying of the epoxy resin in powder form onto a preheated steel pipe which has been blast cleaned.
Fusion bonded epoxy coatings are especially popular for pipeline protection because of their excellent anti-corrosion properties, good adhesion to metal surfaces and resistance to cathodic disbondment from the metallic substrate. However, when used in isolation, fusion bonded epoxy coatings are prone to handling damage during pipe installation and also exhibit relatively high moisture permeation. It has therefore been found that additional protective layers must be used with fusion bonded epoxy coatings for maximum usefulness. A preferred protective layer is a polyolefin outer sheath, polyolefins having many of the qualities lacking in fusion bonded epoxy coatings, such as superior impact resistance, as well as improved impermeability to moisture and many chemicals, as described in U.S. Reissue Pat. No. 30,006 (Sakayori et al). Polyolefins are also easy to fabricate for coating. However, because of their non-polarity, polyolefins bond poorly with metallic substrates. Even the use of adhesives, such as copolymers, in bonding the polyolefin to the metallic substrate has not been found to provide a coating with equal properties to the epoxy/metal bond described above in terms of resistance to hot water immersion and cathodic disbondment.
Examples of multilayer coatings utilizing both a fusion bonded epoxy layer and a polyolefin layer are described in U.S. Pat. Nos. 4,048,355 (Sakayori, et al); 4,213,486 (Samour, et al); 4,312,902 (Murase, et al); 4,345,004 (Miyata, et al); 4,481,239 (Eckner); 4,685,985 (Stucke); 4,519,863 (Landgraf et al); 4,510,007 (Stucke); 4,501,632 (Landgraf); 4,451,413 (Stucke et al); and 4,386,996 (Landgraf et al). Most of these coatings are three-layer systems consisting of an epoxy primer, a copolymer adhesive and a polyolefin outer sheath. Two-layer systems consisting of an epoxy primer and an unmodified polyolefin top coat have not been successful due to poor bonding between the layers. Therefore, the basic principle in the three-layer systems is the use of an adhesive middle layer to provide the bonding agent between the epoxy primer and the polyolefin outer sheath.